Light emissive materials incorporating quinolinolato metal complexes

ABSTRACT

Novel emissive materials suitable for organic light emitting devices are disclosed having a structure (L 2 M) n -X wherein L is a 2-alkyl-8-quinolinolate or 2-phenyl-8-quinolinolate ligand, M is a trivalent metal atom, n is an integer between 1 and 12. When n equals 1, X comprises a monovalent arylate emitter that contains at least one triarylamine group or a light emitting group with emission peak wavelength in the range of 500-750 nm. When n equals 2, X comprises a divalent arylate emitter that contains at least one triarylamine group or a light emitting group with emission peak wavelength in the range of 500-750 nm. When 3≦n≦12, X is an n-valent arylate group.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to light emissive materialsincorporating quinolinolato metal complexes used as emissive materialsin electroluminescent (EL) devices, and in particular organic lightemitting devices (OLEDs). These devices have utility, for example, inflat panel displays.

[0003] 2. Description of the Related Art

[0004] OLEDs are typically comprised of at least a layer of emissivematerial sandwiched between an anode, typically comprised of atransparent conductor such as indium-tin oxide, and a cathode, typicallya low work-function metal, such as magnesium, calcium, aluminum, or thealloys thereof. When a bias is applied across the electrodes, positivecharges (holes) and negative charges (electrons) are respectivelyinjected from the anode and cathode into the emissive layer. The holesand the electrons form excitons in the emissive layer which emit light.Hole transport layers and electron transport layers may also be addedadjacent the respective electrodes to facilitate charge transfer.Depending upon whether hole transport or electron transport is favored,the light emissive layer may be located closer to the anode or thecathode. In some instances, the emissive layer is located within thehole transport or electron transport layer. Known arrangements ofelectrodes, hole transport layers, electron transport layers andemissive layers in multilayer structures are disclosed for example in B.R. Hsieh, Ed., “Organic Light Emitting Materials and Devices,”Macromolecular Symposia, 125, 1-48 (1997), which is incorporated hereinby reference.

[0005] Tris(8-hydroxyquinoline)aluminum (AlQ₃) complex is a widelystudied emissive aluminum complex having the following structure:

[0006] AlQ₃ which has a characteristic green emission with a wavelengthof about 535 nm, may be doped with guest emitter compounds to prepare anemissive system having an emission spectrum close to that of the guest.The emitter is energized by direct excitation or by transfer from theAlQ₃ host. Examples of these systems are described in C. W. Tang, etal., J. Appl. Phys., 65, 3610 (1989), which is also incorporated hereinby reference.

[0007] Conventional doping causes certain problems. For example, if theguest compounds do not disperse properly in the matrix, “aggregation”occurs, local areas of high concentration of the guest compound which inturn leads to “quenching,” a phenomenon in which the guest compoundabsorbs energy but fails to emit at its characteristic wavelength ordesired intensity.

[0008] AlQ₃ has also become the prototype for a class of photoemittingmaterials in which quinolinolato metal complexes are bonded to organicgroups. Examples of this class of materials are disclosed in U.S. Pat.Nos. 5,466,392 and 5,924,869, which are also incorporated herein byreference. Some of these materials show promise for use as emissivelayers in OLEDs, exhibiting properties such as good electron transport,photoemission, high thermal stability, solubility and ease ofsublimation. However, these photoemitting materials do not luminesce atwavelengths characteristic of the organic groups bonded to the metalcomplexes. Instead the organic groups merely modify or shift theemission of the metal complex portion of the material. While some of theorganic modifying groups disclosed in the aforesaid U.S. Pat. Nos.5,466,392, and 5,924,869, may have weak emission spectra, most of themare not light emitting at all.

[0009] The inventors herein have discovered materials based on thebonding of modified quinolinolate ligands to light emitting arylates,which exhibit photoemissive and charge transport properties, and whichcan reduce or eliminate the necessity for doping to improve emissionefficiency or color in an organic light emitting device. In preferredembodiments, these materials exhibit emission spectra close to thecharacteristic emission spectra of the contained light-emitting arylate.In these instances, the metal complex serves to activate the emission ofthe light-emitting arylate, without contributing substantially toemission.

SUMMARY OF THE INVENTION

[0010] The invention is a light emissive material suitable for use in anOLED with a structure (L₂M)_(n)-X wherein L is a 2-alkyl-8-quinolinolateor 2-phenyl-8-quinolinolate ligand, M is a trivalent metal atomcomplexed with the ligand to form a conjugated metal complex, and n isan integer between 1 and 12. When n is equal to 1 or 2, X comprises amonovalent or divalent arylate emitter, respectively, that contains atleast one arylamine group, or which is conjugation-isolated from themetal complex. When 3≦n≦12, X is an n-valent light emitting arylategroup.

[0011] In embodiments, when n is equal to 1 or 2, X is a light emittinggroup having an emission in the range of about 500 nm to about 750 nm.Emission at this wavelength is necessary to obtain energy transfer fromthe metal complex. Provided the emitter's characteristic wavelength ofemission is high enough (between about 500 nm and about 750 nm), thecharacteristic emission of the combined complex-plus-emitter will be atthe characteristic emission wavelength of the emitter. In otherembodiments, the complexes according to the invention comprise an amineor alkane functionality isolating the conjugation of the arylate emitterfrom the conjugation of the metal complex.

[0012] The invention in a further aspect is embodied as a multivalentarylate emitter core bonding to 3 to 12 metal quinolinolate complexesaccording to the following structure:

[0013] where “M” is a trivalent metal and “core” is a multivalent moietysuch as:

[0014] This brief summary has been provided so that the nature of theinvention may be understood quickly. A more complete understanding ofthe invention can be obtained by reference to the following detaileddescription of the preferred embodiments thereof in connection with theattached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The ligands used in the present invention are2-alkyl-8-quinolinolate or 2-phenyl-8-quinolinolate ligands having thefollowing structure:

[0016] where R is substituted or unsubstituted branched or straightchain alkyl, such as, without limitation methyl, ethyl, propyl or butyl,or substituted or unsubstituted aryl, such as, without limitation,phenyl. The ligand(s) L are complexed with metal M bonded to lightemitting group X according to the formula (L₂M)_(n)-X. Complexing of themetal atom takes place with the electron density associated with thenitrogen and oxygen atoms of the ligand (coordinate and covalent bondsrespectively). Metal complexes of quinolinolate ligands generally emitin the green region of the spectrum.

[0017] M is a trivalent metal, preferably aluminum.

[0018] “Light emitting group” and “emitter” are used interchangeablyherein to mean any group in the recited combination which exhibitsfluorescent or phosphorescent emission in the visible spectrum uponrelaxation from an excited state. Thus, in the formula (L₂M)_(n)-X, bothL₂M and X are capable of being emitters. In preferred embodiments, themetal complex L₂M, although capable of emission, does not emit lightupon relaxation from the excited state, but transfers energy to thearylate emitter X, and X is substantially the sole emitter. In otherembodiments X is a very weak emitter, and is provided mainly as a coreon which the quinolinolate complexes are attached. In these embodimentsemission of the material is at a wave length of the metal complex.

[0019] The entire group X is referred to as the emitter, even thoughonly a small portion of X contains the functional groups responsible foremissive transition. In every case X contains an arylate group whoseoxygen atom is bonded to the metal. This structure is required to have astable bond to the metal complex. X may also contain bridging groupsbetween the arylate moiety bonded to the metal and the rest of thearylate emitter. Such bridging groups isolate the conjugation of thearylate emitter from the conjugation of the metal complex, causing theemissive material to emit at a characteristic wavelength of the arylateemitter. The arylate bonded to the metal and the bridging groups are notthemselves emissive, nevertheless, they form a part of X, the entiretyof which is referred to as the arylate emitter.

[0020] Examples of isolating groups include —CH₂—, —CH₂—CH₂—, ArOCH₂—,Ar₂N—, R₂N—, Si—CH₂—, and the like.

[0021] In the formula (L₂M)_(n)-X, where n is equal to 1, arylateemitters may include the following:

[0022] While referred to as “emitters” in the context of thisdisclosure, the above arylates do not cause the emissive material(L₂M)_(n)-X to emit at a characteristic wavelength of these groups.These X groups serve as a core on which quinolinolate complexes areattached.

[0023] In the formula (L₂M)_(n)-X, where n is equal to 2, and divalentarylate emitters may comprise the following:

[0024] all of which have emission in the blue or blue-violet region ofthe visible spectrum.

[0025] Examples of an emissive material according to formula(L₂M)_(n)-X, where n equals 2 and X is an arylate emitter incorporatingarylamine groups include the following:

[0026] The methyl groups on the modified quinolinolate ligands can bereplaced by phenyl groups and still remain within the scope of theinvention. Likewise, additional triarylamine groups can be added to thearylate emitter X without departing from the scope of the invention. Ingeneral, triarylamine is a weak emitter. Accordingly, in embodimentswhere the arylate emitter X consists essentially of one or moretriarylamine groups, the function of the triarylamine group(s) isprimarily to impart hole-transport capability to the emissive material.One of ordinary skill in the art will appreciate that AlQ₃ itself haselectron-transport capability. The triarylamine group may also shift thewavelength of the emission of the metal complex, however, the emissionof the emissive material will not be the characteristic triarylamineemission wavelength.

[0027] An example of preparing an emissive material according to formula(L₂M)_(n)-X, where n is equal to 2 and having amine bridging groupisolating the conjugation of the metal complex from the conjugation ofthe arylate emitter, is given in Scheme 1 below.

[0028] Compound 1 is a quinacridone derivative, which can be preparedaccording to methods set forth by Pei-Hua Liu, et al., “LuminescenceProperties of Novel Soluble Quinacridones,” Journal of Photochemistryand Photobiology A: Chemistry 137 (2000) 99-104, incorporated herein byreference. An appropriate Ullman Coupling reaction can be conductedaccording to Bryan E. Koene, et al., “Asymmetric Triaryldiamines asThermally Stable Hole Transporting Layers for Organic Light EmittingDevices,” Chemistry of Materials, Vol. 10, No. 8 (1998), alsoincorporated herein by reference.

[0029] Compound 2 is added to a dry, nitrogen-flushed vessel. Anhydrousmethylene chloride is added and mixture is stirred and cooled in anacetone, dry ice bath. Excess boron tribromide in a methylene chloridesolution is added dropwise, and the solution is heated to roomtemperature and stirred overnight. Solution is then removed and washedthree times with sodium bicarbonate solution. The aqueous wash is thenwashed with methylene chloride twice, which is then added to the initialorganic solution. The resulting solution is dried over calcium chloride,and reduced to dryness by rotary vacuum. Compound 3 is the crudeproduct.

[0030] After purification, Compound 3 is used in the final reaction.0.005 mole of Aluminum isopropoxide and 0.005 mole of2-methyl-quinolinol is placed in a beaker along with 50 mL of anhydroustoluene. The mixture is heated and stirred until the bulk of the solidsare dissolved. Solution is removed from heat, filtered, and placed in anitrogen-filled flask. A solution of compound 3 (0.0022) mole and 0.005mole of 2-methyl-8-quinolinol, dissolved in hot anhydrous toluene, isadded to the flask.

[0031] The entire mixture is refluxed for 5 hrs, then allowed to cooland stirred overnight. Precipitate is filtered off and washed withethanol and ether. Product is then dried under vacuum.

[0032] In the product 4, an amine functionality is interposed betweenthe arylate group bonded to the aluminum atom, and the remainder of thequinacridone. This amine functionality serves to isolate the conjugationof the metal complex from the conjugation of the quinacridone.

[0033] An example of preparing an emissive material according to formula(L₂M)_(n)-X, where n is equal to 3 is given in Scheme 2 below:

[0034] One equivalent of compound 1 is refluxed with 4 equivalents of1,4-dihydroxybenzene, three equivalents of potassium carbonate, andabout 0.1 equivalent of 18-Crown-6 ether in THF. After refluxingovernight, remaining salts can be removed by filtration, and thesolution can be concentrated via rotary evaporation. The concentratedsolution can be added to a solvent such as hexane, and the resultingprecipitate is compound 2.

[0035] After purification, compound 2 is used in the final reaction.0.005 mole of Aluminum isopropoxide and 0.005 mole of2-methyl-quinolinol are placed in a beaker along with 50 mL of anhydroustoluene. The mixture is heated and stirred until the bulk of the solidsare dissolved. Solution is removed from heat, filtered, and placed in anitrogen-filled flask. A solution of compound 2 (0.0016) mole and 0.005mole of 2-methyl-8-quinolinol, dissolved in hot, anhydrous toluene, isadded to the flask.

[0036] The entire mixture is refluxed for five hours, then allowed tocool and stirred overnight. Precipitate is filtered off and washed withethanol and ether. Product is then dried under vacuum.

[0037] The foregoing examples are for purposes of illustration only andare not to be deemed limiting of the invention which is defined by thefollowing claims.

[0038] Compound 4 in Scheme 1 is expected to have the emission of thequinacridone branch, green or green-yellow.

[0039] Compound 4 in Scheme 2 is expected to have the emission of thealuminum quinolinolate complex, in the green-blue region.

We claim:
 1. A light emissive material with a structure (L₂M)_(n)-Xwherein L is a 2-alkyl-8-quinolinolate or 2-phenyl-8-quinolinolateligand, M is a trivalent metal atom complexed with said ligand to form aconjugated metal complex, and n is an integer between 1 and 12, andwherein when n equals 1 or 2, X is a monovalent or divalent arylateemitter, respectively, that contains at least one triarylamine group, orthat is conjugation-isolated from said metal complex, and when 3≦n≦12, Xis an n-valent arylate group.
 2. A light emissive material according toclaim 1, wherein n is 1, and X is an arylate emitter consistingessentially of triarylamine groups.
 3. A light emissive materialaccording to claim 2, wherein X is


4. A light emissive material according to claim 1, wherein X comprisesan arylate linking group bonded to said metal complex, a conjugatedlight emitting group, and an amine or alkane group between said linkinggroup and said conjugated light emitting group, effective to isolate theconjugation of the metal complex from the conjugation of the conjugatedlight emitting group.
 5. The light emissive material according to claim4, wherein said amine or alkane group is —CH₂—, —CH₂—CH₂—, —ArOCH₂—,—Ar₂N—R₂N—, Si—CH₂—.
 6. A light emissive material according to claim 5,wherein X has the following structure:


7. The light emissive material of claim 1, wherein n is 2 and X is anarylate emitter comprising triarylamine groups.
 8. A light emissivematerial according to claim 1, exhibiting hole-transport capability. 9.The light emissive material of claim 7, wherein the light emissivematerial comprises a compound having the following structure:


10. The light emissive material of claim 1, wherein M is aluminum.
 11. Alight emissive material with a structure

where R is phenyl or alkyl, M is a trivalent metal, and CORE is ann-valent emitter.
 12. A light emissive material according to claim 11wherein CORE is selected from the group consisting of:


13. The light emissive material according to claim 11, wherein M isaluminum.